How to increase width of the walls?
There are many different ways. This is one of them (maybe not the best, but should be easy to follow):


Create a cube
Resize the cube to match the size of the pool ( s )
Apply your object's scale( Ctrl-a > Scale)
Go to edit mode (TAB)
Press 3 to switch to "face selection"
Select top face
Inset the face i
Select the inner face and extrude vertically with e + z (so you can create the "hole" in the swimming pool
If you want to change the width of the walls you can select inner faces (check screenshot) and resize them in both X and Y to get your result ( S-x and S-y)